The oscillating jet meter occupies a special position among direct and indirect volumetric meters, because this type of meter can be designed to have no moving parts and thus generally has a long service life. Moreover, meters of this type operate with high measuring accuracy over a relatively wide measuring range.
The mode of operation of the oscillating jet meter is based on the flow pressure oscillating in a plane, which has a vertical component relative to the mass throughflow and thus induces a periodic directional changes in mass flow. Behavior of this type is exhibited by flow systems which, for example, are based on the Coanda effect or other effects known per se. A liquid or gas jet emerging from a sharp-edged outlet into a chamber of larger cross section having a stream divider is unstable. The free jet entrains fluid particles asymmetrically from one of the corners of the chamber because of the abrupt widening in cross section, so that an underpressure is created there and the fluid jet is deflected sideways.
The fluid in the return line, which is in the form of an annular passage, then undergoes a movement which induces a flow pressure and forces the deflected mass jet beyond its central position into the other flow direction. In this way an oscillating fluid jet is then formed, the frequency of which is proportional, within certain limits, to the flow velocity of the fluid, so that by measuring the frequency the throughflow can be determined.
In spite of their simple construction, commercially available oscillating jet meters have certain disadvantages because they are machined from a metal material, such as cast iron or bronze, and this material must not only resist corrosion, erosion and abrasion by the fluid but also must form the pressure-resistant boundary between the meter and the environment. Such differences in function necessarily lead to compromises if the meter has to be made from a single material. Moreover, a machining operation involves relatively high production costs.
Oscillating jet meters are also known, for examples as described in Swedish patent application No. 77 10 745-6, wherein the chambers and passages of the flow system, which accommodate and convey the fluid, take the form of an assembly which is injection-molded from plastic material and in which the cavities of the flow system open towards one face of the assembly have to be sealed off from the environment. This is effected by covering the respective open side of the assembly with a metal plate, whereas a cast component carrying the pipe connection is provided on the other closed face of the assembly, so that the metal parts disposed on either side of the assembly can be connected to one another by screws and the assembly can be clamped between them.
In view of the surface finish of this flow system, which essentially consists of plastic material, an oscillating jet meter of this type does have advantages over other known meters. However, this oscillating jet meter also has certain drawbacks because in the passages and chambers of the flow system which are normally of rectangular cross section, only the lower wall and the two side walls have smooth plastic surfaces and the upper boundary wall has favorable hydrodynamic properties only when the inside of the covering metal plate has been correspondingly machined.
Furthermore, even with the additional use of sealants, sealing difficulties are encountered in meters of this type, particularly when the fluid is at high pressure and high temperature. These meters are also relatively heavy and necessitate high material and production costs.